Hydrogen is a flammable and explosive gas with a wide variety of industrial and scientific uses. It is axiomatic that handling hydrogen requires utilization of robust safety devices since hydrogen is a highly flammable gas at concentrations in air as low as 4% by volume. Well-known industrial uses of hydrogen include the production of basic staple products of the chemical industry such as ammonia and fertilizers derived there from. Other uses include basic alcohols, hydrogen chloride, reduction of ores for the manufacturing of metals, refinery of oil for the manufacturing of petroleum, and the hydrogenation of vegetable oils for margarine and related industries.
Hydrogen is also widely used for space flight applications, for example, hydrogen is used as a component of hydrogen-oxygen blends used in vehicular propulsion systems. Hydrogen gas is also used in the processing of rocket fuel in the aerospace industry. The combustible nature of hydrogen however, makes its detection vitally important.
Hydrogen is also utilized in a variety of metal forming and microelectronic processing steps which are often of extreme importance in device fabrication and metal interconnect processing of multi-level microelectronic devices.
The increase in oil prices also increased the emphasis on the use of fuel cells, which require hydrogen as a fuel in various stationary and mobile applications, for instance, in fuel cells of automobiles.
In these and other applications, hydrogen sensors are employed to monitor the environment around which hydrogen is utilized, to ensure the efficiency, safety and operational integrity of process systems. For such purposes, a number of hydrogen sensors and complex detection methods have been developed and are in common use. About one-half of all the sensors used to measure hazardous gases measure hydrogen. The bulk of these systems utilize as the detector element a Group VIIIB metal element, for example, Ni, Pd, or Pt, heated to catalytically oxidize the hydrogen, with the resulting change in heat load being the measured parameter for determination of the presence of hydrogen.
A variety of these commercially available hydrogen sensors are based on measuring an electrical characteristic across a sensor element and at least four major categories of sensors and associated methods have been identified.
One type of hydrogen sensor is the “catalytic combustible” or “hot wire” sensor (CC sensor) mentioned in the U.S. Pat. No. 6,006,582 to Bhandari, et. al. The CC sensor comprises two specially arranged beads of a catalytic metal or alloy, such as platinum-iridium wire heated to 600-800 degrees Celsius. One bead is coated with a reactive catalyst. In the presence of a flammable gas, the heat of oxidation raises the temperature of the bead and alters the electrical resistance characteristics of the measuring circuit. This resistance change is related to the concentration of all flammable gases, including hydrogen, in the vicinity of the sensor.
Sensors of such “hot wire” type have cross-sensitivity to other easily oxidized materials, such as alcohols and hydrocarbons. Such easily oxidized materials are common components of gases in a semiconductor-manufacturing environment, and often result in the frequent occurrence of false alarms.
Current hot wire sensors require an oxidation reaction for operation, such sensors are unable to detect hydrogen when it is present in inert gas streams or environments which are not of a character to support an oxidative reaction. This is a deficiency of such hot wire sensors and limits their applicability and utility.
The CC sensor has drawbacks. In oxygen deficient environments or above an upper explosive limit, the oxidation process is quenched causing difficulties in measuring. In addition, since the CC sensor is based upon oxidation, and all hydrocarbons have the same response as hydrogen, this makes it difficult to detect hydrogen in environments which also contain hydrocarbons. Further, the CC sensor element is easily contaminated by halogenated hydrocarbons and is susceptible to poisoning by silicones, lead and phosphorous.
Another commonly used hydrogen sensor is a non-porous metal oxide (MO) sensor. The MO sensor element comprises a non-porous metal oxide (such as zirconium dioxide or tin dioxide) sandwiched between two porous metal electrodes. Such electrodes are typically made of platinum. One electrode is exposed to the reference gas, usually air, and the other electrode is exposed to the test gas being detected.
Mobile ions diffuse to both surfaces of the oxide where they may be eliminated by reaction with adsorbed species. In the absence of gas species which can be oxidized (such as, for instance, carbon monoxide or hydrogen), the electrochemical potential of the sensor may be determined by the Nernst equation and is proportional to the partial pressure of oxygen in the test gas only. In order to achieve sensitivity to hydrogen with this device, the platinum electrode is co-deposited with gold. Since gold is a substantially less efficient donor of electrons than platinum, oxidation rates are reduced, equilibrium conditions are not achieved and the sensor becomes sensitive to the composition of the test gas. The electrochemical potential which develops becomes “non-Nernstian”, and is a complicated function of the kinetics and mass transfer associated with all species reacting at the electrode.
Like the CC sensor, the MO sensor has serious disadvantages. The sensor is not hydrogen-specific and all oxidizable gases in the test gas contribute to the sensor signal. The response is relatively slow and it can take up to 20 seconds to reach 50% of maximum signal when exposed to 1% hydrogen in air at flows below 200 standard cubic centimeters per minute (sccm); the recovery time is even slower taking up to 5 minutes to reach 50% of maximum signal when exposed to less than 200 sccm of air. Finally, in order to achieve even these orders of response time, the device must be operated at temperatures above 350 degrees Celsius. Operating at such temperatures, is potentially unsafe and may cause ignition and/or explosion.
Another class of sensors includes metal-insulator semiconductor (MIS) or metal-oxide-semiconductor (MOS) capacitors and field effect transistors, as well as palladium-gated diodes. In general however, these sensors are limited to detecting low concentrations of hydrogen.
Yet another type of sensor is the metal oxide-semiconductor (MOS) sensor which is also known and is mentioned, for instance, in the U.S. Pat. No. 6,006,582 to Bhandari, et. al. The MOS sensor element comprises an oxide, typically of iron, zinc, or tin, or a mixture thereof, and is heated to a temperature of about 150 degrees Celsius to about 350 degrees Celsius. Bhandari et. al. reported that oxygen absorbs on the surface of the sensor element to create an equilibrium concentration of oxide ions in the surface layers.
The original resistance of the MOS sensor is first measured. When certain compounds, such as, for instance, CO, or hydrocarbons come in contact with the sensor, they are adsorbed on the surface of the MOS element. This absorption shifts the oxygen equilibrium, causing a detectable increase in conductivity of the MOS material.
MOS hydrogen sensors have a number of operational deficiencies and are, therefore, unsatisfactory in many respects. They require frequent calibration and their response times are too long (up to 3-5 minutes). Bhandari et. al. noted that the MOS sensors are unsafe and can cause ignition and explosion, and are susceptible to being poisoned with halogenated vapors. Like the CC and the MO sensors discussed above, they are not hydrogen specific. All volatile organic compounds as well as gases containing hydrogen will react with the sensor materials in the sensing elements of these detectors, thereby providing false readings.
Still yet another sensor is the catalytic gate (CG) sensor, the simplest embodiment of which is a MOS structure, where the metal is usually platinum or palladium deposited on an insulator, such as silicon dioxide. Hydrogen dissociates on platinum or palladium and subsequently diffuses into the bulk of the metal. Hydrogen atoms which arrive at the metal-insulator interface, form a dipole layer, polarizing the interface and consequently changing its electrical characteristics. The CG sensor also has serious drawbacks, particularly slow response time when the surface is contaminated. The surface of platinum or palladium is very much susceptible to contamination and poisoning.
There exists no known prior art teaching of a hydrogen-specific sensor, which quickly responds only to hydrogen gas at room temperature and which is not susceptible to poisoning. Yet, as discussed above, such sensor is highly desirable and the need for such sensor, which is also low cost, lightweight and of a miniature size, is acute.
The present invention discloses such a sensor. It therefore is an object of the present invention to provide an improved hydrogen selective sensor and hydrogen sensing methodology overcoming the aforementioned deficiencies of the previously known hydrogen detectors.
Because hydrogen is used in such a wide variety of environments, it is desirable to have a sensor that will be reproducible and specific to hydrogen, even with varying concentration of background gases such as oxygen, water and other contaminants.
It is also desirable to have a solid state sensor, operating at room temperature, that has no moving parts, has a response time on the order of seconds, would operate with minimum power consumption, does not require frequent calibration, and could be used in a hand-held portable instrument.